1. Technical Field
The invention relates to wax imitation candles and more particularly to an imitation candle resistant to cracking at low temperatures.
2. Description of the Problem
Many people find candle light pleasant. The flickering of light and movement of shadows across a floor or on a nearby wall can be almost hypnotically soothing. As a result, candles have remained popular for generations since the invention of more practical electrical lighting, especially for decorative and mood setting purposes.
Consequently, numerous manufacturers have attempted to meet a demand for a candle like luminary using electrical illumination. A now popular imitation candle is taught in International Publication Number WO 03/016783 A1. This imitation candle uses an internal LED as a light source within a solid appearing body. While a classical image of a candle is of a long, thin, tapering rod, which stands upright in a candle stick and which leaves its flame exposed as it burns down, this imitation candle comes as a relatively short to circumference block or cylinder which is self supporting. Such candles commonly leave the outer wall of the candle intact as the candlewick burns down. When this happens, the candle flame is no longer directly visible when viewed from the side. This results in a diffuse, flickering glow visible through the paraffin wall of the candle, which is imitated by the external shape of the imitation candle.
While the imitation candle of WO 03/016783 appears to be a solid body to users it is in fact hollow. This provides space for the installation of batteries, the LED, LED excitation circuitry and possibly light directing internal components. In addition, the contour of the void's internal surface may be chosen for light transmission issues. While the imitation candle can readily be made in plastic, fabricating it in more realistic wax has presented particular problems.
Wax is highly susceptible to compressive and tensile stress. Waxes also tend to have high coefficients of thermal expansion. Differential heating and cooling of sections of a cast wax body introduces stress. Stress tends to be focused along sharp corners and edges of a wax body. Stress can occur during manufacturing and shipping of the wax shell imitation candles when the imitation candles are subjected to rapid cooling or great temperature extremes, respectively. The cavity adds the problem of internal edges, as well as reducing the strength of the body compared to a solid wax body. In addition, the insert on which battery, excitation circuitry and the LED are mounted will typically be constructed by plastic with the wax body being formed in part on the insert body. Wax will typically have a higher coefficient of expansion than the plastic does, which results in additional stress as temperature of the body decreases and contributes further to the problems of the inherent weakness of wax.
Wax bodies, such as candles, are formed by a process of casting. Where it is desired to incorporate a plastic module in the wax body the plastic module may be fixed in position in a mold and hot wax poured around the module, adding wax as earlier poured wax cools and shrinks, until all voids around the module are filled. Alternatively, a wax shell can be formed that produces the outer visible surfaces of the candle while leaving a space for the module. After the shell is produced a second pour is done to secure the module in position. The amount of wax in the second pour is less than in the first, with the attendant advantages of quicker cooling and faster production speeds. While true, solid wax candles have reasonable durability to withstand cold temperature induced stress, wax bodies made by either of the foregoing casting techniques have proven highly susceptible to cracking. Thin sections of the casting adjacent the module cool more rapidly than thicker sections. Leading edges of the imitation candle also cool rapidly. These sections of rapid cooling result in differential rates of contraction, which can easily result in formation of a crack to relieve stress. Once such a crack propagates into a thicker section of the body it can become a focal point for other stresses and can extend to encircle the imitation candle body.